Repair a Leaking Pipe Under the Kitchen Sink

Kitchen sink leaks happen in plain sight but often go ignored until water pools on cabinet floors and damages what's stored below. The space under your sink is cramped, awkward, and usually packed with cleaning supplies, which makes the problem feel worse than it is. The truth is that most leaks live in one of three places: a compression fitting that's worked loose, a corroded section of old copper or galvanized pipe, or a supply line that's lost its seal. You don't need a plumber for this. You need a wrench, a bucket, and twenty minutes of focused work. The repair changes slightly depending on what's leaking, but the sequence is always the same: shut off water, locate the exact source, drain the line, and then either tighten or replace the bad section. The stakes are straightforward—water damage spreads fast under cabinetry, and a small drip becomes a cabinet replacement if you ignore it. Doing this right means the leak stops, the cabinet stays dry, and you avoid the panic call.

1. Kill Water Flow First. Locate the shutoff valves under the sink—usually one each for hot and cold water supply. Turn both valves clockwise until they stop. They should be finger-tight; don't force them. If the valves are corroded or won't turn, shut off the main water supply to the house instead.
2. Bleed the Pressure Out. Turn on the kitchen faucet (both hot and cold) to bleed air and water from the lines. Let it run until nothing comes out. This prevents a gush of water when you disconnect fittings.
3. Catch the Water Now. Position a five-gallon bucket or large container directly under the leaking joint or pipe section. This catches trapped water in the line and keeps your cabinet floor from getting soaked during disassembly.
4. Find the Culprit. Dry everything under the sink with a clean cloth. Look carefully at every visible joint: supply line connections to shutoff valves, shutoff valves to the wall, P-trap connections, and any visible copper or galvanized pipe. Feel for moisture and look for mineral deposits or green corrosion around fittings. Mark the leak source with a dry-erase marker or tape.
5. Tighten First. If the leak is from a compression fitting (the hexagonal nut where supply line meets the shutoff valve), use a wrench to tighten it by turning clockwise. Hold the valve body steady with a second wrench so you don't twist the pipe. Turn no more than one-quarter turn; over-tightening cracks the fitting. If water stops, you're done. If it still leaks after tightening, move to replacement.
6. Unbolt the Bad Piece. Use an adjustable wrench or pipe wrench to disconnect the leaking section at both ends. For compression fittings, turn counterclockwise. For threaded joints, turn counterclockwise until hand-tight, then finish with the wrench. Remove the old section and lay it aside. You may need to hold the valve or pipe body with a second wrench to prevent rotation.
7. Inspect Before Reassembly. If you disconnected a fitting with exposed threads, wipe them clean with a cloth and inspect for damage. If the threads are crushed or corroded, the fitting won't seal properly when reconnected. For compression fittings, look at the ferrule (the metal or plastic ring under the nut). If it's dented, pitted, or cracked, it needs replacement.
8. Cut the New Line. Measure the length of the old pipe or supply line you removed. If replacing with supply line (braided steel or rubber), cut to the same length with a utility knife or hose cutter. If replacing rigid pipe (copper or PVC), cut with a tubing cutter for copper or a hacksaw for PVC, deburring the cut end with a file. Measure twice before cutting.
9. Seal the Threads Right. If reconnecting threaded joints (galvanized or brass), wrap the male threads with plumber's thread seal tape, turning the tape clockwise around the threads. Wrap three to four layers. For compression fittings, slide the original ferrule onto the new pipe end, or install a new ferrule if the old one was damaged. Do not use tape on compression fitting threads—only on NPT (tapered) threads.
10. Thread and Tighten Gently. Install the new pipe or supply line, threading it into or onto the existing valves and connections. Start all connections hand-tight, then use a wrench to tighten compression fittings by one-quarter to one-half turn. For threaded joints, tighten by hand plus one-quarter turn with a wrench. Use two wrenches again—one on the nut or fitting, one on the valve body—to avoid stressing the connection.
11. Watch for Twenty Minutes. Slowly turn both shutoff valves counterclockwise to restore water flow. Open the faucet at the sink and let water run for ten seconds. Close the faucet. Dry everything under the sink with a clean cloth and watch for twenty minutes. Check the bucket you placed—it should be bone dry. Inspect every connection you made or tightened.
12. Dry and Restore. If the test is clean, remove the bucket, dry the cabinet interior with a cloth, and return your cleaning supplies and stored items to their original places. If water got on wood shelving, wipe it dry and leave the cabinet doors open for two hours to allow air circulation. Discard any cardboard boxes or paper that got wet.